“The captain says that the frost giants threw all these rocks out here when they were having a battle with old Njord, the god of the sea,” she said.  Then, as she caught sight of a lighthouse on a low outer ledge,—­“Why, Father!” she cried, “I thought we were going to stop at every lighthouse on the coast.”

“So we are, after we leave the Skaergard,” replied Lieutenant Ekman.  “I came down as far as this several weeks ago when the ice went out of the fjord.  There are two or three months when all this water is frozen over and there can be no shipping; but as soon as the ice breaks up, the lamps are lighted in the lighthouses and I come down to see them.  Now it is so light all night that for two months the lamps are not lighted at all unless there is a storm.”

Gerda ran to the rail to wave her handkerchief to a little girl on the deck of a lumber vessel which they were passing.

“The lighthouse keepers have a good many vacations, don’t they?” she said when she came back.

“Yes,” replied her father; “those on the east coast of Sweden have several months in the winter when the Baltic Sea and the Gulf of Bothnia are covered with solid ice; but on the south and west coasts the lighthouses and even the lightships are lighted all winter.”

“Why is that?” questioned Birger, coming to join them.

“There is a warm current which crosses the Atlantic Ocean from the Gulf of Mexico and washes our western coast.  It is called the Gulf Stream.  This current warms the air and makes the climate milder, and it keeps the water from freezing, so that shipping is carried on all winter,” Lieutenant Ekman explained.

Just then a sailor came to tell them that their dinner was ready.  While they were eating, the launch made a landing at the first of the lighthouses which the inspector had to visit.

While their father was busy, the twins clambered over the rocks, hunting for starfishes and sea-urchins, and Gerda picked a bouquet of bright blossoms for their table on the boat.

At the next stopping-place, which was Gefle, the captain took them on shore to see the shipyard where his own launch, the North Star, was built; and so, all day long, there was something to keep them busy.

As the boat steamed farther north, each new day grew longer, each night shorter, until Birger declared that he believed the sun did not set at all.

“Oh, yes it does,” his father told him.  “It sets now at about eleven o’clock, and rises a little after one.  You will have to wait until you cross the Polcirkel and get to the top of Mount Dundret before you have a night when the sun doesn’t even dip below the horizon.”

“We must be pretty near the Arctic Circle now,” exclaimed Gerda.  “It is growing colder and colder every minute.”

“That is because the wind is blowing over an ice-floe,” said her father, pointing to a large field of ice which seemed to be drifting slowly toward them.